Conventionally, polycrystalline silicon has widely been used as a material for forming gate electrodes of metal oxide semiconductor (MOS) devices. However, as such a MOS device becomes minuter, there is a fear that driving current will fall because of an increase in the resistance of a polycrystalline silicon gate electrode or the occurrence of a depletion layer. To avoid these problems, in recent years an attempt to form a gate electrode by using a metal material in place of polycrystalline silicon has been made.
With a metal gate electrode formed by using a metal material, the work function must be controlled for controlling threshold voltage. Accordingly, the method, for example, of forming metal gate electrodes of an n-channel MOS transistor (nMOS transistor) and a p-channel MOS transistor (pMOS transistor) by using different metal materials has traditionally been adopted.
In recent years the method of changing a work function by doping a metal material with nitrogen (N) has also been proposed (see IEEE Electron Device Letters, Vol. 25, No. 2, p. 70 (February 2004)). In addition to these methods, the following methods, for example, are proposed. When complementary metal oxide semiconductor (CMOS) transistors having metal gate electrodes are fabricated, layers are formed in an nMOS transistor and a pMOS transistor by using a same metal material. One region is masked and the other region is doped with nitrogen. By doing so, the metal gate electrode of the nMOS transistor differs from the metal gate electrode of the pMOS transistor in work function (see Japanese Unexamined Patent Publication No. 2000-31296). Layers are formed in an nMOS transistor and a pMOS transistor by using a same metal material. After the whole of the layers are doped with nitrogen, nitrogen in one layer is made to out-diffuse. The other layer is covered so that nitrogen will not out-diffuse. By doing so, a metal gate electrode of the nMOS transistor differs from a metal gate electrode of the pMOS transistor in work function (see Japanese Unexamined Patent Publication No. 2005-79512).
However, if different metal materials are used for forming metal gate electrodes of an nMOS transistor and a pMOS transistor, the metal gate electrodes of the NMOS transistor and the pMOS transistor must be formed separately. As a result, a manufacturing process and device structure are complex compared with conventional devices. In addition, manufacturing costs rise.
If a same metal material is used for forming metal gate electrodes of an nMOS transistor and a pMOS transistor and the work functions of the metal gate electrodes are controlled by adjusting the dosage of nitrogen, the resistance of the metal gate electrodes becomes high, depending on their structure. In addition, a manufacturing process becomes complex. Such problems must be solved. Moreover, a proper work function difference which can practically be used for CMOS transistors is not obtained between metal gate electrodes of an nMOS transistor and a pMOS transistor and a method for obtaining a proper work function difference between metal gate electrodes of an nMOS transistor and a pMOS transistor is not yet devised.